Coprecipitation of phosphate with calcite: Molecular-scale evidence for incorporation and inclusion mechanisms

Coprecipitation of phosphate in calcium carbonate minerals is a ubiquitous geochemical phenomenon in marine sedimentation and cave stalagmite formation, however, it is not clear whether phosphate is incorporated into the calcite structure. In this research, we applied solid-state nuclear magnetic resonance (NMR) spectroscopy to analyze P speciation during coprecipitation with calcite. The ³¹P NMR results show three peaks with chemical shifts of 3.9, 3.0 and −1.0 ppm, indicative of at least three phosphate species in the coprecipitates. Combined with advanced ³¹P{¹H} cross-polarization (CP)/MAS, ¹H DE/MAS, ³¹P{¹H} 2-d heteronuclear correlation (HetCor) and ³¹P{¹³C} cross-polarization rotational echo double resonance (CP-REDOR) NMR experiments, the 3.9 ppm peak can be tentatively assigned to calcite structural defects as amorphous calcium phosphate (ACP)-like environments while the 3.0 ppm peak arises from a carbonated hydroxyapatite (CHap). The ³¹P NMR peak at −1.0 ppm can be assigned to structurally incorporated phosphate in the calcite crystals in the form of HPO₄²⁻. Nano secondary ion mass spectrometry (NanoSIMS) and high-resolution scanning transmission electron microscopy (HR-STEM) analysis further suggests that the incorporated HPO₄²⁻ substitutes for the structural carbonate group (CO₃²⁻) of calcite. However, the local expansion stress field generated with HPO₄²⁻ incorporation in the calcite structure prevents PO₄/CO₃ isomorphous substitution and favors the precipitation of calcium phosphates. The findings of this study not only provide deep insights into carbonate crystal chemistry but also shed light on the application of carbonate materials as potent geochemical proxies in paleoenvironmental reconstructions.